In this project, designed to investigate the biological properties of thymosin polypeptides, our results indicate that thymosin fractions can generate helper or suppressor T cells. We have found that as little as a 30 minute preincubation of spleen cells from normal mice with thymosin, in the absence of antigen, can generate helper T cells capable of augmenting the anti SRBC response 50-150%. A 2 hour preincubation of BSA-separated nude mouse spleen cells with thymosin fraction 5, alpha 1 or alpha 7 can induce expression of Ly 1 ion, 2 ion, 3 ion cells. When these cells were injected into TxBM mice (thymectomized, irradiated, bone marrow reconstituted mice) as a source of T cells, fraction 5 and alpha 1 induced helper T cells, augmenting the response to SRBC ten fold. Thymosin alpha 7 induced cells capable of completely suppressing the responses generated by fraction 5 and alpha 1. We plan to further evaluate active thymosin polypeptides in biological assay systems, measuring augmentation or regulation of the immune response, and to develop other animal assay systems for measuring helper and suppressor function that are more directly applicable to estimating thymosin's function in humans with aberrant immunological systems.